1. Technical Field
This invention is directed to an electrically heated (i.e., heatable) vision unit, for example an electrically heated windshield of a motor vehicle. An electrically heated vision unit may be defogged and deiced by application of an electric current to an electrically conductive film on or in the transparent substrate of the vision unit. More particularly, the invention is directed to an electrically heated vision unit having a new and advantageous arrangement of an upper bus bar in contact with the upper edge of the electrically conductive film.
2. Background Art
Systems are known for heating vision units such as architectural glass, motor vehicle windows and the like. A preferred technology, now in commercial use, employs an electrically conductive film or coating covering most of the area of a transparent substrate, for example an inside surface of one of the two glass plys in a typical motor vehicle laminated windshield. The electrically conductive film is substantially transparent to radiation in the visible wavelength range. Exemplary of such technology is that disclosed in U.S. Pat. No. 4,543,466 issued on Sep. 24, 1985 to Ramus, entitled "Bus Bar Arrangement for Uniformly Heating a Trapezoidally-Shaped Electrically Heated Windshield", which is assigned to the assignee of the present invention and the disclosure of which is incorporated herein by reference. Therein an electrically heated windshield is disclosed to comprise an electrically conductive layer interconnecting a horizontally extending upper bus bar and a horizontally extending lower bus bar. To reduce costs and avoid undue manufacturing complexity, the electrically conductive wires from the vehicle electrical system to the two bus bars typically are run bundled together to the bottom of the windshield. There, a first of the wires is connected to the lower bus bar and the second wire, to complete the circuit, is connected to an electrically isolated conductive path extending in the windshield to the upper bus bar. Specifically, such conductive path extends horizontally below the lower bus bar to one side of the windshield and from there vertically to the upper bus bar.
The lower bus bar of the electrically heated windshield of the Ramus patent is wider (i.e., has greater lateral dimension) than the upper bus bar. This is quite common in electrically heated windshields, because motor vehicle windshields are typically wider at the base and the bus bars (both upper and lower) extend across substantially the entire lateral dimension of the windshield. Thus, the right-hand and left-hand ends of the lower bus bar extend beyond the upper bus bar. During heating of the windshield, since the upper bus bar is less wide, it carries a higher current density than the lower bus bar. The current density tends not be evenly distributed across the upper bus bar. Rather, the right-hard and left-hand end portions have a much higher current density, since they provide a disproportionate amount of electrical power to the corresponding extended end portions of the lower bus bar. As a result, so called "hot spots" develop at the end portions of the upper bus bar. This is disadvantageous, since overall performance would be improved if electrical current density (and hence heating) were more evenly distributed.
It is an object of the present invention to provide improved electrically heated windshields and more particularly, to overcome the aforesaid "hot spot" problem in present electrically heated windshields and in vision units generally wherein one bus bar extends beyond the other. These and other object and advantages of the invention will be better understood in the light of the following disclosure.